1. Field of the Invention
The present invention relates to position detection systems and more particularly, to a position detection system utilizing interferometric and centroid optical measuring to obtain straightness and angular measurement of payload position for a cartesian robot used in near-field antenna measurement.
2. Description of Related Art
High performance antennas are increasingly prevalent in the art as spacecraft, aircraft, ship and ground vehicle mission requirements become more sophisticated. One problem in the development and manufacture of high performance antennas is the measurement of antenna performance. Traditionally, antenna measurement was conducted by placing the antenna at a remote location, and measuring the amplitude and phase response characteristics of the antenna in its operational range. Typical operational distances for high gain antennas can range from 50 feet to 3 miles. This measurement technique, known as far-field testing, suffers from significant limitations, such as susceptibility to weather effects, ground reflections and increasing real estate costs.
As an alternative to far-field testing, near-field testing was developed. A near-field test is conducted in an indoor test range using a microwave probe to sample the field radiated near the antenna under test (AUT). A computer collects the amplitude and phase data sampled by the microwave probe, and calculates the far-field equivalent response using a Fourier transform technique.
To make accurate near-field measurements, all the significant antenna energy must be sampled by the microwave probe. Highly directive antennas, such as reflectors and waveguide phased arrays, beam most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field scanner is utilized. Precision cartesian robots are used to move the microwave probe along a planar pattern approximately normal to the antenna aperture. To accurately reconstruct the measured field, the probe must sample points at some minimum spacing based on the Nyquist sampling theorem. This distance is usually less than half the wavelength of the antenna signal (.lambda./2). Therefore, to achieve an accurate near-field measurement, the precise position of the microwave probe and its planarity with respect to the AUT is critical.
Obtaining high accuracy position information for the microwave probe has proven to be difficult to achieve. Undesired variations in the microwave probe position can occur due to flexure of the rails of the cartesian robot which support the microwave probe. One solution to this problem is to utilize an extremely rigid structure for the cartesian robot rails. For example, precision-lapped, natural-close-grained granite rails have been utilized due to their extremely rigid characteristics. The granite surface remains quite stable with time due to its natural formation and machining processes, and has a third of the thermal expansion coefficient of steel. However, the weight of such a heavy duty structure makes it increasingly difficult to provide a portable test system. Such portability can be of significant benefit to test antennas at remote locations. Moreover, the high cost of manufacturing and transporting such a heavy duty structure renders it impractical for many remote near-field test applications.
Since relatively light weight and inexpensive near-field test structures are susceptible to position errors due to flexure, an alternative to granite construction is to accept the undesired variation in microwave probe position so long as these variations can be accurately determined. Information regarding the variations to the microwave probe position can then be used in correcting the far-field equivalent response, or to manipulate the microwave probe to the correct position.
One way to provide microwave probe position information in light weight systems is to incorporate optical skeletons into the cartesian robot system. An optical skeleton utilizes laser beams which are projected collinearly with the x and y-axis rails of the robot. Accurate distance measurement can be obtained by conventional laser interferometry systems which measure changes in displacement of the microwave probe along the x and y-axes. Laser interferometers measure range by determining the phase difference between two interferometer paths.
Straightness can be measured by use of photodetectors, such as lateral effect photodiodes, which measure the relative motion of a laser beam centroid. Separate laser beams are generally utilized for straightness measurement and for distance measurement, resulting in relatively complex optical skeleton systems. Laser interferometer techniques have also been utilized to measure straightness, but these techniques further add complexity and expense.
Thus, a critical need exists to provide an accurate position and straightness measuring system for a cartesian robot which enables the use of a lightweight, portable and inexpensive near-field test system. It would be particularly desirable to take maximum advantage of known measuring technologies, while reducing the complexity and associated cost of conventional optical skeleton systems.